Internal vs External Forum - Making distinctions between the two help us understand that not living up to the sacramental signs of matrimony that are meant to reflect the love of Christ for his Church, does not necessarily mean that a rupture with friendship with God (mortal sin) has occurred.

Also, if I may add, matrimony between one man and one women as a visible sign of our faith in the Trinity as another example of the external forum giving witness to our faith in the Triune God, if unable to be witnessed may not necessarily mean the mentioned couple is in a state of mortal sin.

This distinction between the Internal & External forum is necessary to understand the distribution of priest from one parish to another after they are accused of pedophilia, regardless of if they are guilty or not.